Skip to main content

Advertisement

Distinct S-wave reflectors (bright spots) detected beneath the Nagamachi-Rifu fault, NE Japan

Abstract

Distinct phases reflected from mid-crustal reflectors (SxS and PxP phases) were observed in seismograms of aftershocks of 1998 M5.0 Sendai earthquake at nearby stations. We estimated the locations of the reflectors (bright spots) by using arrival time differences between these phases and direct waves. A clear reflector is located in the depth range of 15 to 21 km just beneath the fault plane of the M5.0 event. It dips toward the NNE direction with a dip angle of about 25°. Other reflectors are also located beneath the fault plane of the event. Internal structure of the S-wave reflector was estimated from spectral amplitude ratios of reflected SxS-wave to direct S-wave. Observed spectral ratios show that S-wave velocity in the reflector body is 1.1 km/s, and its thickness is about 50 m. This suggests that a thin reflector body (bright spot) partially filled with fluids exists in the lower crust beneath the focal area of the 1998 event.

References

  1. Ansell, J. H. and E. G. C. Smith, Detailed structure of a mantle seismic zone using homogeneous station method, Nature, 253, 518–520, 1975.

  2. Asano, Y., N. Umino, A. Nakamura, T. Okada, S. Hori, T. Kono, K. Nida, T. Sato, A. Hasegawa, M. Kosuga, and A. Hasemi, Spatial distribution of seismic scatterers beneath the Ou Backbone range, northeastern Japan, estimated by seismic array observations, J. Seism. Soc. Jpn., 52, 379–394, 1999.

  3. Hasegawa, A., A. Yamamoto, N. Umino, S. Miura, S. Horiuchi, D. Zhao, and H. Sato, Seismic activity and deformation process of the crust within the overriding plate in the northeastern Japan subduction zone, Tectonophys., 319, 225–239, 2000.

  4. Hori, S., N. Umino, Y. Asano, and A. Hasegawa, S-wave reflectors in the crust of northeastern Japan, Programme and Abstracts The Seismological Society of Japan, 1999 Fall Meeting, p. 140, 1999.

  5. Horiuchi, S., A. Hasegawa, A. Takagi, A. Ito, M. Suzuki, and H. Kameyama, Mapping of a melting zone near Mt. Nikko-Shirane in northern Kanto, Japan, as inferred from S × P and S × S reflections, Tohoku Geophys. J., 31, 43–55, 1988.

  6. Inamori, T., S. Horiuchi, and A. Hasegawa, Location of mid-crustal reflectors by a reflection method using aftershock waveform data in the focal area of the 1984 Western Nagano Prefecture earthquake, J. Phys. Earth, 40, 379–393, 1992.

  7. Matsumoto, S. and A. Hasegawa, Distinct S wave reflector in the midcrust beneath Nikko-Shirane volcano in the northeastern Japan arc, J. Geophys. Res., 101, 3067–3083, 1996.

  8. Nakajima, J., T. Matsuzawa, A. Hasegawa, and D. Zhao, Three-dimensional structure of Vp, Vs and Vp/Vs beneath northeastern Japan: Implication for arc magmatism and fluids, J. Geophys. Res., 106, 21843–21857, 2001.

  9. Nakamura, A., Y. Asano, and A. Hasegawa, Estimation of deep fault geometry of the Nagamachi-Rifu fault from seismic array observations, Earth Planets Space, 54, this issue, 1027–1031, 2002.

  10. Okada, T. and A. Hasegawa, Activity of deep low-frequency microearthquakes and their moment tensor in northeastern Japan, Bull. Volcanol. Soc. Jpn., 45, 47–63, 2000 (in Japanese with English abstract).

  11. Umino, N. and A. Hasegawa, Three-dimensional Qs structure in the northeastern Japan arc, Bull. Volcanol. Soc. Jpn., 37, 217–228, 1984 (in Japanese with English abstract).

  12. Umino, N., T. Okada, and A. Hasegawa, Foreshock and aftershock sequence of 1998 M5.0 Sendai, northeastern Japan, earthquake and its implications for earthquake nucleation, Bull. Seism. Soc. Am., 2002 (in press).

Download references

Author information

Correspondence to Norihito Umino.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Keywords

  • Fault Plane
  • Focal Mechanism
  • Bright Spot
  • Uppermost Mantle
  • Arrival Time Difference